There has been a growing interest in configuring SXA in an existing installation.
- Can a non-SXA site coexist with SXA sites?
- Same content tree?
- Can non-SXA sites share content with SXA sites?
Yes that is possible.
I created two sites in one sitecore instance. One is SXA based and the other one is on Habitat. These sites are on Sitecore 8.2 update-3 instance with SXA 1.3. As such there wasn't any conflict other than the controller name that should be different from the SXA components for example if you need to build an accordion then the controller name should be different than AccordionController as it is in used by SXA accordion renderings.
You may also face some issue with Castle.Core if you're using GlassMapper. For which you have to define assembly binding for latest castle (castle.core and castle.windsor) assemblies in web.config.
Rest is all good in deploying sxa and non-sxa sites in same Sitecore instance.